The present invention relates to cycloconverters in general, and more particularly, to a cycloconverter that is designed to work into an active load.
The initial work on cycloconverters was done in Germany during the 1930's using mercury arc tubes to convert 50 Hz power to 16 2/3 Hz power for traction motors on electric railroads. While considerable developmental work has been carried out, the practical application of cycloconverters has, until very recently, been restricted by excessive cost of equipment.
The principal application, or proposed application, of cycloconverters has been in the role of frequency conversion for power systems. The essential element of a cycloconverter is the pairing of phase-controlled rectifiers in inverse-parallel with modulation of the triggering to achieve an alternating output; the frequency of the output is usually lower than the source frequency. In general, the classical employment of cycloconverters has been in converter roles which can be catagorized according as the input or the output frequency is variable or constant. The four possible combinations with examples of their application are enumerated below.
1. Constant frequency input and output: One of the earliest applications was that of the German work on traction motors mentioned above. PA1 2. Variable frequency input, constant frequency output: Generating systems for aircraft represent an important application of this class. PA1 3. Constant frequency input, variable frequency output: Such systems find application in various types of variable speed motor drives. PA1 4. Variable frequency input, adjustable frequency output: This would be applicable to vehicle propulsion from an on-board prime mover; it would permit the prime mover to be operated in its efficient speed range and allow the vehicle to travel at any desired speed.
An essential feature of all these earlier applications is that they all operate into passive loads; the cycloconverter provides the power that serves the load from the voltage sources that supply the cycloconverter.
Cycloconverters may be classified according as they are commutated naturally by the a.c. supply voltage or are forcecommutated by external sources. The present invention relates to cycloconverters of the naturally commutated type.
In the preferred embodiment of the present invention the voltage waveform that is synthesized in the cycloconverter provides an insignificant fraction of the total power to the load; a source external to the cycloconverter provides the power to the load. The supply voltages for the cycloconverter can be derived from the external source that supplies power to the load or from another source. The load current flows through the cycloconverter, but, since this is driven by forces external to the cycloconverter this current tends not to be correlated to the voltage waveform synthesized in the cycloconverter. It is principally in this respect that the present invention stands in marked contrast to all prior art relating to cycloconverter technology.
The extent of this departure from conventional practice may readily be appreciated on considering the application for which this invention was made. In this first application a cycloconverter is used to synthesize a single-phase voltage waveform which is superimposed upon the voltages of the three phases of an electric power distribution feeder. This superimposed voltage is impressed by injecting it between the common point of the wye-connected secondary of the substation transformer and the grounded neutral that serves all phases of the distribution feeder. The superimposed voltage is used merely to add to the normal supply voltage a signal to represent digital information at the points served by the distribution feeder.
The current that flows through the cycloconverter is in this case the neutral current of the distribution feeder. This is determined by the unbalance of the loads among the three phases and accordingly its phase is totally unpredictable. In fact, this neutral current contains significant contributions of third harmonic and accordingly it may depart significantly from a simple 60 Hz wave of unknown phase.
The voltage waveform added to the three phase voltages must be in a well-defined phase relationship to the voltages that supply the power to the feeder. This requires that the input voltage wave to the cycloconverter be derived from the voltages that supply the feeder and that unknown phase shifts in the cycloconverter, per se, be avoided. It is this latter consideration for avoiding unknown phase shifts within the cycloconverter that requires the use of an envelope type of cycloconverter for the particular application for which this new technology was devised.
One should observe, however, that this new technology could be applied to cycloconverters in which unknown phase shifts occur. The consideration of importance is the amount of phase shift that could be tolerated; if instead of injecting a single phase voltage in the neutral one had elected to impress individual signaling voltages on each phase then some uncertainty in phase shift in the superimposed signal could have been tolerated.
Another very important consideration with respect to the new technology is related to the fail-safe provisions that must be incorporated in the design of the cycloconverter. In the application mentioned above the cycloconverter appears as a conductive element in the neutral of the electric distribution feeder. All the neutral current must pass through the cycloconverter or through protective bypass circuitry. In the event of a fault on the distribution feeder currents of magnitudes approaching 10,000 amperes may be encountered in the neutral and it is imperative that current continuity be maintained so that excessive voltages will not arise between phase and neutral on the distribution feeder. Failure to provide for this fail-safe action could result in excessive voltages being delivered to the customers under certain types of fault condition with the consequence that equipment on the customers's premises could be destroyed.
When the cycloconverter provides the power to the load as in the case of prior applications, the current waveform is determined by the waveform synthesized in the cycloconverter and the impedance of the load. In this invention the current waveform is not determined by the waveform produced by the cycloconverter and the current zero-crossings are not correlated with the zero-crossings of the supply voltage of the cycloconverter. This necessitates a firing technology that differs markedly from that of established practice. The new firing technology and the protective or fail-safe circuitry are additional elements of novelty in this invention.